This invention relates to multi-element antenna arrays for the transmission and reception of radio waves having a directional characteristic. In particular, it relates to those arrays whose direction of maximum transmission or reception can be altered or "steered" by electrical switching means, and which are commonly known as "phased arrays".
It is well known that an antenna array consisting of a number of separate radiating antenna elements which are simultaneously driven from a common source of radio frequency power, through an electrical power dividing and an electrical phasing network, can be so arranged in spaced and the individual phases so determined, that the radiated energy will be highly concentrated in one direction and strongly suppressed for other directions.
Such a combination of multiple antennas is known as a "phased array". Because of the particular arrangement of the individual antennas in space, combined with a particular set of electrical phases at each element, the individually radiated waves combine and add together in phase in the preferred direction. In other non-preferred directions, the vector sum of the radiated waves from all of the antenna elements will be very much weaker and in some cases may completely vanish.
It is also well known that an array of antennas, fixed in position, can have its preferred radition direction altered or "steered" by changing the relative electrical phases of the radio-frequency (RF) energy supplied to each element. To accomplish this, RF switches are usually employed which change the phase relationships among the multiple elements. When this is done, the complete array and its associated power dividing, switching and phasing networks constitute a "steerable phased array". Such arrays have been used for RADAR antennas at UHF and microwave frequencies and for communications at radio, HF, VHF, and UHF frequencies.
It is also well known that any radio antenna, or any interconnected array of antennas, has identical directional characteristics when acting either as a transmitter or as a receiver of radio waves, to or from distant points. In this disclosure, we will be discussing transmitter radiation characteristics in most cases, but it is to be understood that the directional characteristics apply equally well to an application as a receiver.
This invention is a new form of steerable phased-array antenna which, in one embodiment, uses four vertical antenna elements above the plane of the earth, equally spaced on a circle parallel to the earth, arranged to radiate outward parallel to the earth's surface. When combined with power-dividing, switching, and phasing networks which are here disclosed, it is possible to maximize the radiation in any one of four primary directions without moving the antenna. The angular width of the radiation pattern is sufficiently wide that the four possible patterns overlap, allowing transmission or reception in any horizontal direction, over 360.degree. of azimuth angle around the horizon.
Application of such an antenna is advantageous for radio communications to and from a station which must communicate with one or another of various distant stations at various times, which lie in different directions.
Examples of prior art in array antennas are discussed in the following paragraphs.